_loadFileConfigs: parameter to override CONFIG_DIR

To load configs in subdirectories from modules _loadFileConfigs needs
argument to point into directory. Sample modules with own configs will
look like this:

```javascript
// Loading global configs for application
var config = require("config");
// Load local configs for module
config._loadFileConfigs("./configs");

module.exports = function() {
    // Use local and global defaults!
};
```